Field of the Invention
The present invention relates to a process for producing biomethane suitable for supplying a natural gas network that incorporates a process for providing heat for heating the biogas production step; the process comprises at least steps of producing biogas by anaerobic fermentation of organic matter, of pretreating and compressing the biogas and also of permeation in order to obtain, after a first separation, a stream of biomethane and a gaseous permeate having a reduced methane content; at the same time, the process provides the heat necessary for the anaerobic fermentation step.
Related Art
Biomethane is a methane-rich gas obtained via a suitable purification from biogas, and has the same characteristics as natural gas for which it may be substituted.
Added more recently to the upgrading—mainly on-site or nearby—of biogas is that of this biogas purified to the specifications of natural gas. The biomethane produced may thus be used as a non-fossil substitute for natural gas, supplementing natural gas resources with a renewable portion produced at the heart of territories. It can be used for exactly the same purposes.
In particular, within the context of the upgrading thereof, biomethane—a renewable substitute for natural gas that has the same properties as the latter—may be injected into a natural gas distribution or transport network that makes it possible to connect gas producers and consumers.
A natural gas distribution or transport network makes it possible to supply consumers with natural gas. The network is maintained at a pressure between 2 and 6 bar for distribution, 15 and 25 bar for medium-pressure distribution and 25 and 80 bar for transport.
As for biogas, it is a gas produced by the natural or controlled fermentation of plant or animal organic matter (methanization). The characteristic component of biogases is methane, which is formed during the biochemical degradation of organic waste, the other main constituent is carbon dioxide. A biogas also contains, but in smaller proportions, water, nitrogen, hydrogen sulphide, oxygen, and also other organic compounds, in trace amounts.
Depending on the organic matter and the techniques used, the proportions of the components differ, but on average a biogas comprises, as dry gas, from 30% to 75% methane, from 15% to 60% CO2, from 0 to 5% nitrogen, from 0 to 5% oxygen and trace compounds.
Biogas is produced by methanization of organic matter, that is to say by anaerobic fermentation. It is produced in a sealed tank also referred to as a methanizer or digester. It is necessary to operate in the absence of air (anaerobic process) and to maintain a stable temperature in the chamber of the reactor. This temperature depends on the methanization process, that is to say on the type of bacteria used for the degradation of the organic matter, but in all cases it will be necessary to provide heat, thus for a mesophilic process the temperature should be maintained between 30° C. and 37° C., whilst a thermophilic process requires a temperature of from 50° C. to 55° C.
Thus, since the continuous addition of organic matter at a temperature lower than that of the methanizer and also the outside temperature contribute to the cooling of said methanizer, it is necessary to make provision for providing heat for the biogas production step. The heat to be provided may thus represent from 5% to 20% of the energy contained in the biogas produced.
At the same time, in order to be able to benefit from subsidized rates for the purchase of biomethane during the injection to the network, it is necessary for the source that provides the heat for the methanization to use renewable energy.
In order to address this twofold problem, the heat source most commonly used is the biogas produced by the digester. In this case, the heat is provided to the methanization process by heat exchange with boiler water, this being heated using biogas withdrawn before purification to the biogas produced. The portion of biogas thus withdrawn before purification does not therefore participate in the final production.
Yet, in order to be able to upgrade their production in the form of substitute natural gas in a worthwhile manner, biogas producers must be able to have the largest possible production since (i) the purification of biogas is expensive in terms of investment and (ii) it is necessary to be able to provide sufficient amounts of biomethane in order to have outlets—very particularly in the case of small productions.
Being able to use the largest possible portion of biogas to produce biomethane is therefore essential, and even imperative for a small producer whose biogas production may be of the order of twenty Nm3/h to several tens of Nm3/h. For these small producers, being able to use the largest possible portion of the methane produced, up to 99% of the methane produced or even more, may be a major advantage.
At the same time, in order to benefit from subsidized rates for the purchase of biomethane during the injection to the network, it remains imperative to utilize a heat source that uses renewable energy.
Solutions exist that propose to recover available heat produced by the purification unit (heat of the compressor or of the cold units) but the supply of heat from this source is not sufficient for the total requirements of the digester (it only covers around 20% of the requirements).
Another known solution consists in recovering the heat produced by a system for destroying vented gases via a low-GCV boiler burning a biogas containing between 10% and 20% methane, preferably 15% methane, or by thermal oxidation which is applicable for gases containing between 2% and 8% methane, preferably 5% methane. However, these solutions for destroying vented gases are expensive, too expensive for small projects. Moreover, they only make it possible to provide the methanizer with a small portion of the heat needed (the solution covers at most 15% of the requirements).
Therefore, no solution exists to date that makes it possible to provide the methanizer with all of the heat needed which simultaneously:                is derived from the biomethane production process and therefore uses renewable energy,        keeps all of the biogas production available for purification,        does not involve large additional costs, which would be excessive for a small producer and therefore does not require expensive supplementary treatments in order to provide the means for heating the methanizer.        